1. Field of the Invention
A security document and method of producing it The present invention relates to security documents such as bank notes, identity cards or the like, with multilayer security elements having a layer in which diffraction structures, in particular holographic structures, are embossed in the form of a relief structure and which are combined with a reflective layer, and to a method for producing the same.
2. Discussion of Related Technology
Optically variable elements such as holograms, diffraction grids or interference layer elements have been preferably used for some time as protection against forgery or copying due to their optical properties that vary with the viewing angle. For mass production of such elements it is customary to produce so-called master holograms which have the particular phase information in the form of a three-dimensional relief structure. Starting with the master hologram one produces by duplication so-called press dies for embossing the required holograms in large numbers of units.
The embossing can also be done directly on the document material as described in EP-A 0 338 378. In a continuous process bank note paper in a roll form is first printed on both sides and then provided in certain areas with a holographic structure. The lacquer to be embossed and the relief structure are simultaneously transferred to the paper by covering the surface structure of the press die with a radiation-curable lacquer. As soon as paper and press die are brought in contact the lacquer is cured. The lacquer now adheres to the paper surface and has the holographic relief structure. Then the embossed structure is given a thin vacuum metalized layer that permits the holographic information to be observed in reflection.
Since paper is virtually impermeable to UV radiation the curing of the lacquer can in this case only take place with the aid of electron radiation, a very elaborate and expensive method that furthermore damages the paper. For this reason the production of embossed holograms directly on the document material has not become accented in practice, although this procedure has great advantages with respect to resistance to forgery since the hologram is connected virtually undetachably with the substrate. Due to the much more cost-effective production and more versatile applicability embossed holograms are therefore usually prepared as multilayer elements on a separate carrier and transferred to the document by means of an adhesive layer. The layer structure is dimensioned, or prepared by additional measures, in such a way that the hologram can be removed from the carrier layer after being glued to the document.
The multilayer element applied to the carrier material can be produced e.g. by the method known from U.S. Pat. No. 4,758,296. A matrix in web form wound on rolls is provided with a liquid resin and brought in contact with a plastic carrier material. The liquid resin is simultaneously cured by UV or electron radiation. In a further step the relief structure is provided with a thin metal layer so that the hologram can be observed in reflection. To be transferred to a document the layer structure is finally provided with a hot-melt adhesive layer that is activated under the action of heat and pressure.
However this security element has the disadvantage that the hologram element might be detached from the document by reheating the hot-melt adhesive, and transferred to another.
In general, so-called transfer embossing foils have more than the layers described in U.S. Pat. No. 4,758,296. For example EP-A 0 170 832 describes a transfer embossing foil comprising a carrier material, a first layer of lacquer permitting subsequent detachment of the carrier material, a second layer of lacquer in which the diffraction structures are embossed, a metal layer and a layer of bonding agent. Such a foil can be glued to a document by the method known from EP-A 0 433 575. The embossing foil in which the hologram structure is embedded is applied to a document locally in the form of a marking. For this purpose the document is printed at a certain place with an adhesive which only becomes viscous and sticky through UV, gamma or electron radiation. This activation takes place either before or after the transfer foil and document are brought together.
Although this security element offers irreversible adhesion to the document since the cured adhesive is not reactivable, the embossed structure can be exposed if the layer bordering the relief structure or the metal layer has a different chemical base.